Automatic brightness control
The contrast control is also linked, to some extent, with the brightness setting and often needs readjustment when the brightness control is varied. The brighter the room the greater is the need to advance the settings of both brightness and contrast controls for a bright picture and effective reproduction of the various shades of grey in the scene.A circuit which automatically accomplishes this is known as ‘automatic brightness control’ (A.B.C.). It forms a part of the video amplifier circuitry.
Automatic Brightness control Circuit:
- A typical circuit arrangement is illustrated in Fig. 1.1.
- The LDR is mounted on the front panel of the cabinet to sense changes of illumination in the room.
- It forms a part of the video amplifier circuit that controls the screen grid voltage of the video amplifier tube and the bias voltage (grid to cathode potential) of the picture tube.
- As shown in Fig. 1.1 resistor R1 (which is in parallel with the LDR) and resistor R2 form a potential divider across the B supply to feed necessary dc voltages both at G2 of V1 and control grid of the picture tube.
- When illumination in the room increases, say, when lights are switched on or during daylight, the resistance of the LDR decreases.
- Because of the potentiometer arrangement this increases the screen grid voltage to tube V1.
- The increased screen grid voltage provides greater contrast because of enhanced amplification of the composite video signal.
- At the same time the increased potential also appears at the grid of the picture tube.
- This decreases the net dc negative bias, between the grid and cathode of the picture tube, which in turn results in increased screen brightness.
- For a decrease in light intensity the LDR resistance increases and the resultant fall in the video amplifier screen grid and picture tube grid voltages decrease both contrast and brightness in proportion to changes in the general illumination of the room.
Figure 1.1 Automatic brightness control circuit.